There is increasing concern for the risk of hypernatremic dehydration in infants breastfeeding poorly. It is important to differentiate normal weight changes as infants adapt to extrauterine life from excessive weight loss from breastfeeding failure or mismanagement. We review recent data on normal weight changes in exclusively breastfeeding infants and those at risk for hypernatremic dehydration to help health professionals determine when infants require further scrutiny and supplementation. The data suggest that perinatal practices influence infant weight changes. Protecting normal birth and early initiation of breastfeeding should reduce the incidence of excessive weight loss and risk of hypernatremic dehydration